Federal Opposition leader Malcolm Turnbull.
Federal Opposition leader Malcolm Turnbull. AAP

Turnbull says free vote on gay marriage 'likely'

A FREE vote on gay marriage is "likely" but when it will happen and whether it will be successful, remains to be seen, says senior Liberal Minister Malcolm Turnbull.

Questioned over last week's High Court decision to strike down the ACT'S same-sex marriage laws, Mr Turnbull told Sky News on Sunday he believed Prime Minister Tony Abbott could allow a conscience vote before the next Federal Election.

The long time supporter of gay marriage said that given the opportunity he would vote in favour of amending the marriage act but said he could not predict whether the majority of his colleagues would do the same.

Deputy Opposition Leader Tanya Plibersek has called on Mr Turnbull to co-sponsor a private members bill, calling for the vote.

Mr Turnbull said his position as a member of cabinet prevented him from co-sponsoring the bill but admitted Australia was falling behind the countries it was "culturally closest to.

"So people of the same sex can get married in Auckland and Wellington, Toronto and Ottawa and Vancouver, in New York and Los Angeles, and Baltimore, in Cape Town, but not Australia," Mr Turnbull said

"It does start to look as if we're the ones out of step."